Depending on your intentions this may be a duplicate of Mathematica DestructuringMathematica Destructuring but if not:
# + #2 - #3 & @@ {1, 3, 5}
Or
# + #2 - #3 & @@ # & @ {1, 3, 5}
And
Fold[
Function[{x, y}, If[y < 5, {# ~Join~ {y}, #2}, {#, #2 ~Join~ {y}}] & @@ x],
{{}, {}},
Range[10]
]
Similar to Rojo's answer (and my answer to Mathematica DestructuringMathematica Destructuring):
Fold[
# /. {a_, b_} :> If[#2 < 5, {a ~Join~ {#2}, b}, {a, b ~Join~ {#2}}] &,
{{}, {}},
Range[10]
]